A mechanism regarding the redox reaction in lanthanum cerium oxide (LaxCeyOz) post-deposition annealed in reducing and oxidizing atmosphere was schematized and discussed in association with the presence of lanthanum as a substitutional cation. Analyses have been performed using X-ray diffraction, energy-filtered transmission electron microscopy, scanning transmission electron microscope-energy dispersive spectroscopy line scan, and capacitance-voltage measurements. The results showed the presence of an oxygen vacancy when La(3+) was in its substitution site, while annihilation of oxygen vacancy was accompanied by a displacement of La(3+) from the substitutional site to the interstitial site via a kick-out mechanism prior to its disappearance from the CeO2.